Large, high pressure fluid swivels are commonly used in offshore hydrocarbon floating production systems. Such a fluid swivel may have a sealing surface diameter of between two and eight feet (0.6 to 2.5 meters) and carry fluid at a working pressure of over 1,000 psi (70 mega Pascals, or 70 MPa). In such a system, hydrocarbons such as oil and natural gas as well as other fluids, are transferred between an undersea well and a ship, the fluid swivel allowing the ship to weathervane (turn with changing winds, waves and currents). Such fluid swivels typically include inner and outer ring-shaped swivel parts rotating on one another. An annular chamber is formed between the swivel parts, and a pair of gap passages extend from the chamber to the environment. One or more seal devices lie along each gap passage, in a ring-shaped cavity of a swivel part, to prevent the leakage of pressured fluid into the environment. Each seal device includes a seal formed of low friction soft (relative to engineering metals) material and a harder backup ring on the downstream side of the pressure seal to support it.
One source of problems with fluid swivels of the type described above, arises from extrusion of the dynamic side of the soft seal material into the "extrusion gap" formed between the backup ring and the sealed surface region that is sealed against. Such extrusion results in seal failure. Extrusion is avoided by keeping the gap between the backup ring and sealed surface very small. U.S. Pat. No. 4,602,806 describes an arrangement where the backup ring can slide toward the sealed surface under the pressure of fluid in the swivel, which minimizes the extrusion gap. Improvements in systems where the backup ring or other corresponding member can be pushed toward the sealed surface be an increasing force as higher pressure fluid is applied to the swivel, would be of value.